Ah I see, the だった comes "after" the noun. Thanks Ringil, that makes much more sense. Could Da be described as the informal form? Ive heard that desu makes a sentence more polite.
– Master YodaSep 19 '18 at 20:49

2 Answers
2

Part of the confusion appears to arise from the fact that your two "sentences" are actually incomplete.

私の友達は綺麗な人

私の友達は綺麗だった人

These are fragments: they are only phrases, not full sentences. They could end in the copula (だ for plain-form familiar speech, です for polite speech), or they could end in something completely different:

私の友達は綺麗な人 [をかみ殺して食べる。] - My friend [bites to death and then eats] beautiful people.

私の友達は綺麗だった人 [をかみ殺して食べる。] - My friend [bites to death and then eats] people who were beautiful.

Even with the copula, the meaning is slightly different for the second one:

私の友達は綺麗な人 [だ・です] - My friend [is] a beautiful person.

私の友達は綺麗だった人 [だ・です] - My friend [is] a person who was beautiful.

If a な adjective is in the past tense, the な is replaced with だった, and the meaning parses out to "was [ADJ], used to be [ADJ]".

If you just mean that the past tense should apply to the whole statement, you would use the past tense for the final copula, but leave the adjective in the regular attributive form (using the な):

Thank you! I really like your answers, you put a lot of effort into making sure I fully understand the concept and even sometimes helping me see the errors in my questions. I appreciate that.
– Master YodaSep 19 '18 at 22:58

But ending word です also has past tense form.
Because i-adjective + でした is still under polemical circumstance, I would like to avoid touching this matter now.
For na-adjective, below patterns are possible.
先月が一年中で一番穏やかだったです。
先月が一年中で一番穏やかでした。